Lean bike up against a wall, non-crank side facing you, so one of the handlebars is barely touching. Make sure it is standing straight up and down. If the bike is leaning towards the wall, the scale of the frame will be distorted.

Using a tape measurer, pull out a long section and place the base of it on the ground next to your bike. With a small piece of tape, secure the tape measurer to the bike frame. Make sure the tape measurer is straight up and down, from the ground to the bike frame. Again, any leaning will distort the scale.

Stand back about 8 feet, crouch down so you are eye level with the center of the bike, hold your phone sideways, and snap a picture of the bike. Make sure you are eye level with the bike and not standing over top of it, or taking a photo from a bottom angle. The frame should look as 2D as possible.